This invention relates to radiation curable unsaturated polyester coating compositions, processes for coating a surface with such compositions, and the resultant coated articles. Radiation cure has become more and more popular in industry due to its high cure speed, and relativly low or no VOC. (Meth)acrylate monomers and oligomers are the most prevelant radiation curable compositions, and are typically cured in the presence of free radical initiators and electron beam or ultraviolet light (UV) radiation. In recent years, cationic systems have been developed for cationic cure, these systems are based on cycloepoxide, vinyl ethers. Cationic systems are not inhibited by oxygen, therefore no nitrogen blanket is needed.
Radiation curable compositions based on unsaturated polyesters and vinyl ethers have been disclosed, for example by Friedlander, et al, U.S. Pat. No. 5,536,760, assigned to PPG Industries, Inc., wherein radiation curable compositions based on unsaturated polyesters and compounds having at least two vinyl ether groups are shown; U.S. Pat. No. 5,334,456 which disclosed free radical curable compositions based on saturated backbone with terminal maleate or fumarate and vinyl ethers; U.S. Pat. Nos. 5,334,455 and 5,340,653, which disclosed saturated backbone with maleate or fumarate terminal groups and vinyl ethers; Jonsson, et al., U.S. Pat. No. 5,446,073, assigned to Fusion Systems Corporation, which disclosed UV pliotopolyinerization processes employing a charge transfer complex without a photoinitiator.
U.S. Pat. Nos. 5,340,653; 5,334,456; and 5,334,455, assigned to Stamicarbon, B. V., teach free radical radiation curable compositions comprising vinyl ether compound containing 1 to 6 vinyl ether groups; maleate and/or fumarate end group-containing urethane polymer. The maleate/fumarate can be a half ester with epoxy functional compound, or the maleate/fumarate can be end groups on a saturated backbone. The object of the aforementioned Stamicarbon patents is flexibility compared to the aforementioned PPG Industries patent. Although "vinyl ether" would not be understood to those skilled in the art as including propenyl ethers, the Stamicarbon patents define their vinyl ethers by a structural formula which would encompass propenyl ethers; however, no working examples using propenyl ethers were presented in the Stamicarbon patents.
Propenyl ethers have been developed or cationic cure, but have not been specifically suggested for free radical, radiation cured copolymerization with internally unsaturated polyester coating compositions. U.S. Pat. Nos. 4,749,807; 4,845,265; 5,286,835 disclosed vinyl ether terminated ester oligomers which included propenyl ethers; U.S. Pat. No. 4,775,732 disclosed vinyl ether terminated ester and urethane resins. U.S. Pat. No. 5,153,237 described urethane based propenyl ethers for cationic cure; Crivello has disclosed some propenyl monomers in U.S. Pat. No. 5,567,858 based on cyclic ethers, polyethers and alkanes. Radiation curable compositions have been disclosed in U.S. Pat. No. 5,055,357. Although propenyl ethers have been developed in recent years discussed above, they have been mainly used in the cationic polymerizations as disclosed in U.S. Pat. Nos. 5,567,858, 5,055,357; 5,286,835; 4,845,265; and 4,775,7/32. Propenyl ethers were mentioned as a possible electron donor for the formation of charge transfer complexes for photopolymerization in U.S. Pat. No. 5,446,073, however, due to the steric effect of the methyl group in the propenyl ethers, the real cure rate is dependent on the electron acceptor. With an unsaturated polyester as electron acceptor, no practical cure rate can be obtained without photoinitiator according to the U.S. Pat. No. 5,446,073.
Propenyl ethers are typically mixtures of different isomers, each propenyl ether group has two possible isomers depending on the methyl group configuration. Due to the steric effect of the methyl groups in the propenyl ether molecules, they are expected to be much less reactive than their vinyl ether analogs, and therefore in most cases vinyl ethers are preferred as in the aforementioned Stamicarbon patents. Prior to this invention, it was expected that no practical cure rate call be obtained with propenyl ethers.